Presently, X-ray diffraction analysis for substances is widely used in many fields. For example, it can be used to measure structure of crystalline substances (e.g., phase analysis), structural changes of crystalline substances (e.g., residual stress measurement), etc. During X-ray diffraction analysis in the prior art, an X-ray tube with a material Cu, Cr, Fe, or Mo as the anode target are often used. However, the characteristic X-ray emitted from such an X-ray tube is of long wavelength and therefore can only achieve a depth of penetration not greater than 10−4 m in some materials such as Mg, Al, or Si. As a result, X-ray diffraction analysis is only applicable to the surface of a test sample or work piece made of such a material, up to the present.
In view of the drawback that the penetration capability of X-ray is not so strong due to a long X-ray wavelength, patent CN1049496C discloses an “X-Ray Residual Stress Measuring Device and Method”; wherein, the device in that patent employs a X-ray tube emitting a short-wavelength characteristic X-ray and high tube voltage modified based on that of an existing X-ray residual stress measuring device, and the receiving slit is a position-restricting receiving slit that only receives diffracted rays from a measured part in a test sample or work piece. In the X-ray diffraction measuring method disclosed in the patent, short-wavelength characteristic X-rays are employed based on a measuring method for the existing device; the measured part is placed at the center of a goniometer circle, and only the diffracted rays from the measured part is permitted to enter into the radiation detector due to the position-restricting receiving slit, while the diffracted rays and scattered rays from any other part of the work piece are shielded, so that the residual stress at any part in the measured work piece can be measured within the range of penetration depth of the X-rays. By displacing the work piece, the X-ray stress analyzer can measure the residual stress at another part in the work piece; in that way, determination of three-dimensional distribution of residual stress in a work piece made of a material such as beryllium alloy is achieved. In addition, due to the fact that the shorter the X-ray wavelength is and the lower atomic number of one or more elements for the irradiated work piece materials is, the greater the penetration depth of the incident X-ray will be, diffracted rays from different depths and different positions in an even thicker work piece can be detected with such a method.
However, since the X-ray stress analyzer or the X-Ray Residual Stress Measuring Device employs an X-ray diffraction back reflection method to collect a diffraction pattern, the X-ray propagates over a longer path in the work piece. Furthermore, the intensity of the emergent ray from the work piece becomes more attenuated as the propagation path length increases, the principle being shown in FIG. 1, in which the intensity of an emergent ray corresponding to an incident X-ray with an intensity I0 after passing through a work piece with a linear absorption coefficient α and a thickness t is I=I0×e−αt. It can be seen from the above formula, the intensity of an emergent X-ray not only depends on the properties of the work piece and the intensity of the incident ray, but also varies with the propagation distance in the work piece; the longer the propagation distance is (i.e., the longer the sum of the propagation path lengths of the incident ray and diffracted ray in the work piece), the more the intensity loss will be, and thereby the lower the intensity of the emergent ray (the intensity of the diffracted ray) will be. A decreased intensity of the diffracted ray will cause a reduced SNR of measurement and thereby an adverse effect on the measurement of X-ray diffraction. As a result, the technique in the above patent cannot take full advantage of the strong penetration capability of a short-wavelength X-ray, resulting in a lower measurable depth inside of a measured work piece.